1. Field of the Invention
The present invention relates to monolithic piezoelectric actuators including piezoelectric ceramics and methods for manufacturing such piezoelectric actuators. The present invention particularly relates to a piezoelectric actuator including an active section and inert sections including dummy internal electrodes, and also relates to a method for manufacturing such a piezoelectric actuator, the active section being disposed between the inert sections and distorted by the application of an electric field.
2. Description of the Related Art
Piezoelectric actuators have been used to move magnetic heads or print heads for inkjet printers. The following actuator has been proposed: a monolithic piezoelectric actuator that is prepared by an internal electrode-ceramic co-firing process so as to have a small size.
Japanese Unexamined Patent Application Publication No. 2001-352110 (Patent Document 1) below discloses an exemplary monolithic piezoelectric actuator. FIG. 4 is a perspective view of the piezoelectric actuator disclosed in Patent Document 1.
The piezoelectric actuator 101 includes a ceramic sintered body 102 made of a piezoelectric ceramic. The ceramic sintered body 102 is prepared by an internal electrode-ceramic co-firing process. In the descriptions below, the direction along the thickness of each piezoelectric ceramic layer sandwiched between internal electrodes is defined as a thickness direction. In the ceramic sintered body 102, an active section 103 is located at the approximate center of the ceramic sintered body 102 and inert sections 104 and 105 are located at both sides thereof in the thickness direction.
In the active section 103, a plurality of first internal electrodes 106 and second internal electrodes 107 are alternately arranged in the thickness direction with each piezoelectric ceramic layer disposed therebetween. The first and second internal electrodes 106 and 107 extend perpendicularly to the thickness direction of the ceramic sintered body 102.
The ceramic sintered body 102 has a first side surface 102a and includes insulating layers 108 extending on the first side surface 102a in a lateral direction perpendicular to the thickness direction. The insulating layers 108 cover portions of the first internal electrodes 106 that are exposed at the first side surface 102a. Furthermore, the ceramic sintered body 102 has a second side surface 102b opposed to the side surface 102a and includes insulating layers 109 extending on the second side surface 102b. The insulating layers 109 cover portions of the second internal electrodes 107 that are exposed at the second side surface 102b. 
In the inert section 104, three dummy internal electrodes 111 to 113 are arranged in a striped pattern in the same level. Dummy internal electrode groups each including the three dummy internal electrodes 111 to 113 are disposed between ceramic layers. In the inert section 105 and the inert section 104, a plurality of other dummy internal electrode groups each including dummy internal electrodes 114 to 116 are disposed between ceramic layers.
The dummy internal electrodes 111 to 113 have a stripe shape and are electrically isolated from each other. The dummy internal electrodes 114 to 116 also have a stripe shape and are electrically isolated from each other.
An external electrode 117 extends vertically on the second side surface 102b of the ceramic sintered body 102. In the active section 103, the external electrode 117 is connected to the internal electrodes 106 exposed at the second side surface 102b. 
A similar external electrode, which is not shown in FIG. 4, extends on the first side surface 102a. The external electrode extending on the first side surface 102a is electrically connected to the second internal electrodes 107. Therefore, the active section is driven by applying a voltage between the external electrode 117 and the external electrode on the first side surface 102a such that the monolithic actuator is operated.
In the inert sections 104 and 105, the dummy internal electrodes disposed between the ceramic layers are not supplied with a voltage. Thus, the inert sections 104 and 105 do not function as actuators.
A conventional monolithic piezoelectric actuator having a configuration in which inert sections are located on both sides of an active section has a problem in that a sintered body is cracked and/or the inert sections are separated from the active section because of the difference in sintering behavior between the active section and the inert sections. On the other hand, in the piezoelectric actuator 101 disclosed in Patent Document 1, the dummy internal electrodes 111 to 113 and 114 to 116 included in the inert sections 104 and 105 are arranged at the same levels and are separated from each other. Thus, the sintered body is prevented from being cracked and/or the active section 103 is prevented from being separated from the inert sections 104 and 105 due to the difference in sintering behavior therebetween.
In the piezoelectric actuator 101, it is necessary to reduce the influence of a support structure on distortion. In this case, the inert sections 104 and 105 usually have a large size in the thickness direction such that the active section 103, which is distortable, is spaced from a support section for mechanically supporting the piezoelectric actuator 101 at a large distance. This causes the ceramic sintered body 102 to have a large thickness. If the number of the internal electrodes included in the active section 103 for driving the piezoelectric actuator is increased such that a large distortion is created, the ceramic sintered body 102 has a large thickness.
When the ceramic sintered body 102 has a large thickness, the ceramic sintered body 102 prepared by an electrode-ceramic co-firing process the thickness of the ceramic sintered body 102 is not uniform.
In applications in which monolithic piezoelectric actuators are used, in electronic device applications in particular, the size of the piezoelectric actuators must be strictly controlled.
Therefore, when the thickness of the ceramic sintered body 102 is not uniform, the upper surface 102c and lower surface 102d of the ceramic sintered body 102 must be ground such that the ceramic sintered body 102 has a uniform thickness. If the size of portions that are ground as described above is large, the dummy internal electrodes 111 to 113 and 114 to 116 may be exposed at the ground surfaces. This possibility increases with a reduction in the distance between the dummy internal electrodes, that is, a reduction in the thickness of the stacked piezoelectric ceramic layers. The exposure of the dummy internal electrodes 111 to 113 or 114 to 116 at the upper surface and lower surface of the ceramic sintered body 102 may cause mounting defects if the piezoelectric actuator 101 is mounted on a printed circuit board. The piezoelectric actuator 101 having the exposed dummy internal electrodes 111 to 113 or 114 to 116 is a defective product, and therefore, must be eliminated. This reduces the yield.